Re: [PATCH v3] x86/uaccess: Use pointer masking to limit uaccess speculation

[Josh Poimboeuf]

On Mon, Sep 14, 2020 at 09:21:56PM +0200, Borislav Petkov wrote:
> On Mon, Sep 14, 2020 at 11:48:55AM -0700, Dan Williams wrote:
> > > Err, stupid question: can this macro then be folded into access_ok() so
> > > that you don't have to touch so many places and the check can happen
> > > automatically?
> > 
> > I think that ends up with more changes because it changes the flow of
> > access_ok() from returning a boolean to returning a modified user
> > address that can be used in the speculative path.
> 
> I mean something like the totally untested, only to show intent hunk
> below? (It is late here so I could very well be missing an aspect):
> 
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 2bffba2a1b23..c94e1589682c 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -7,6 +7,7 @@
>  #include <linux/compiler.h>
>  #include <linux/kasan-checks.h>
>  #include <linux/string.h>
> +#include <linux/nospec.h>
>  #include <asm/asm.h>
>  #include <asm/page.h>
>  #include <asm/smap.h>
> @@ -92,8 +93,15 @@ static inline bool pagefault_disabled(void);
>   */
>  #define access_ok(addr, size)					\
>  ({									\
> +	bool range;							\
> +	typeof(addr) a = addr, b;					\
> +									\
>  	WARN_ON_IN_IRQ();						\
> -	likely(!__range_not_ok(addr, size, user_addr_max()));		\
> +									\
> +	range = __range_not_ok(addr, size, user_addr_max());		\
> +	b = (typeof(addr)) array_index_nospec((__force unsigned long)addr, TASK_SIZE_MAX); \
> +									\
> +	likely(!range && a == b);					\

That's not going to work because 'a == b' can (and will) be
misspeculated.

-- 
Josh